1. Field of the Invention
The present invention relates to a frequency doubler which makes use of coherent light, for use in the field of optical data processing, or in the field of applied optic measurement control, and also relates to a short wave laser source and an optical data processing apparatus, and also relates to a method of manufacturing a frequency doubler.
FIG. 16 is a structure of a conventional short wave laser source which is composed of a semiconductor laser 21 and a frequency doubler 22, in which a fundamental wave P1 emitted from the semiconductor laser 21 passes through lenses 24, 25 and a half-wave plate 26 and then enters into a waveguide 2 formed in the frequency doubler 22. The fundamental wave P1 thus having entered is transmitted through the wave guide in a TM.sub.00 mode which is a lowest order mode, and is converted into the TM.sub.00 mode which is a lowest order mode of high harmonic waves which are used for a short wave laser beam radiated from the frequency doubler.
Referring FIG. 17, which is a schematic view illustrating a conventional frequency doubler basically using an waveguide, detailed explanation will be made hereinbelow of the generation of a high harmonic wave (wavelength of 0.42 .mu.m) with respect to a fundamental wave having a wavelength of 0.84 .mu.m (refer to "Optics Letters" by K. Yamamoto, K. Mizuuchi, and T. Taniuchi, Vol. 16, No. 15, August 1991, Page 1,156).
As shown in FIG. 17, a waveguide 2 is formed on a LiTaO.sub.3 substrate 1, and a layer 3 whose domain is periodically inverted (domain-inverted structure) is formed in the waveguide 2. The high harmonic wave P2 can be produced with a high degree of efficiency by compensating inconsistency in a propagation coefficient between the fundamental wave P1 and the high harmonic wave P2 to be generated, through the periodical structure of the domain-inverted layer 3 and a domain-non-inverted layer 4. When the fundamental wave P1 is incident upon the entrance surface 10 of the waveguide 2, the high harmonic wave P2 is efficiently produced from the waveguide 2; that is, the substrate 1 having the above-mentioned structure serves as a frequency doubler.
Detailed explanation will be made of a method of manufacturing this conventional frequency doubler with reference to FIGS. 18a to 18d.
Referring at first to FIG. 18a, Ta 6 is periodically patterned on the LiTaO.sub.3 substrate 1 with the use of usual photo-processing and dry-etching. Thereafter, the LiTaO.sub.3 substrate formed thereon with a pattern of Ta 6 is subjected to proton-exchange at a temperature 260 deg. C. for 50 minutes so as to form proton-exchange regions 7 having a thickness of 0.8 .mu.m directly below slits which are not covered with the Ta 6. Next, referring to FIG. 18b, it is heat-treated at 600 deg. C. for 10 minutes. The temperature rising rate for the heat-treatment is set to 50 deg. C./min. Accordingly, domain-inverted regions 3 are periodically formed since the proton exchange regions 7 have reduced Li so that the Curie temperature is low in comparison with the LiTaO.sub.3 substrate 1; thereby the domain can be partly inverted. Then, etching is made with the use of a mixed solution of HF and HNF.sub.3 the ratio of which is 1:1, for two minutes, so as to remove the Ta 6. Further, a waveguide is formed in the above-mentioned domain-inverted regions with the use of proton-exchange, and thereafter, as shown in FIG. 18c, Ta as a mask for the waveguide is patterned in a stripe-like manner so that a slit having a width of 4 .mu.m and a length of 12 mm is formed in the Ta mask. The substrate covered with the Ta mask is subjected to proton-exchange at a temperature of 260 deg. C. for 16 minutes so as to form a proton-exchange layer 5 having a thickness of 0.5 .mu.m. Next, referring to FIG. 18d, the substrate is subjected to heat-treatment at a temperature of 380 deg. C. for 10 minutes after the Ta mask is removed. An area directly below the slit in the protective layer having been subjected to the proton-exchange is turned into the waveguide 2 having a refraction index increased by about 0.03. In this case, the crystallizability of LiTaO.sub.3 is deteriorated so as to loose a non-linear optical effect during the proton-exchange for forming the waveguide. That is, the high refractive index layer 5 which has been at first subjected to the proton-exchange has lost its non-linear optical effect. However, the proton exchange layer 5 is enlarged by the heat-treatment so as to be turned into the waveguide 2, and in this condition the non-linearity is substantially recovered.
With the frequency doubler manufactured by this conventional method, a TM.sub.00 mode which is the lowest order mode of the high harmonic wave P2 can be obtained, having an output power of 2.4 mW and a conversion efficiency of 2.4%, with respect to the TM.sub.00 mode which is the lowest order mode of the fundamental wave P1 having a wavelength of 0.84 .mu.m and propagating through the waveguide 2 if the length of the waveguide 2 is set to 9 mm, and the power of the fundamental wave P1 is set to 99 mW. In this case, the reduced conversion efficiency is 24 %/W.
With the use of a 10 mW semiconductor laser as a short wave laser source constituted by the above-mentioned frequency doubler as shown in the figures, 70 mW of the laser beam enters the frequency doubler from which 1.2 mW of blue laser light is then obtained as a short wave laser beam.
Next, explanation will be made of another conventional example with reference to the drawings. In this example, the TM.sub.00 mode of the fundamental wave and the TM.sub.10 mode of the high harmonic wave are coupled together in the waveguide (Refer to "Light Wave Guide Electronics", the Japan Society for the Promotion of Science, Page 88).
FIG. 19a shows an enlarged section of the waveguide and an electric field strength distribution of the fundamental wave and the high harmonic wave. Further, FIG. 19b is a sectional view illustrating the waveguide in the frequency doubler.
As shown in FIG. 19a, the TM.sub.00 mode has one peak of electric field. Meanwhile, the TM.sub.01 mode has two electric field peaks in the thicknesswise direction, and the phases of these peaks are inverted with respect to each other. ZnS 2a which is a nonlinear optical crystal, is formed on a glass substrate 1b, and thereafter, a linear layer 9 made of TiO.sub.2 having nonlinear optical effect is applied thereto by vapor deposition by sputtering. Accordingly, the conversion between the TM.sub.00 mode of the functional wave and the TM.sub.10 mode of the high harmonic wave can be made with a high degree of efficiency. That is, in the case of conversion between the TM.sub.00 mode of the fundamental wave and the TM.sub.01 mode of the high harmonic wave within a nonlinear optical crystal with no linear layer 9, high harmonic wave energies produced on the +E side and -E side of the electric field are canceled by each other, so that substantially no high harmonic wave can be obtained.
With the above-mentioned frequency doubler basically using a waveguide formed in nonlinear optical crystal as mentioned above, in the case of the conversion between the lowest order modes of the fundamental wave and the high harmonic wave, that is, more specifically, in the case of conversion from the TM.sub.00 mode of the fundamental wave into the TM.sub.00 mode of the high harmonic wave, there has been found a problem in which a variation in output power of the high harmonic wave is induced by fluctuation in refractive index caused by optical damage. FIGS. 20a and 20b show the electric field strength distributions and the refractive index distributions of the TM.sub.10 mode of the fundamental wave and the TM.sub.00 mode of the high harmonic wave in the section of the waveguide. In a normal condition in which no optical damage is present, the refractive index of the waveguide part is uniformly higher than that of the LiTaO.sub.3 substrate as shown in FIG. 20a. Although the actual refractive index distribution is graded, a rectangular distribution is used in this explanation for the sake of brevity. The optical damage is likely to occur in a short wave range of, for example, 0.4 .mu.m band; that is, the higher the light intensity, the greater the refractive index decrease. When the TM.sub.00 mode of the high harmonic wave is produced, the refractive index decreases around a peak in the electric field of the TM.sub.00 mode. On the contrary, the refractive index around the TM.sub.00 mode of the high harmonic wave increases. Accordingly, the TM.sub.00 mode of the fundamental wave is shifted toward the substrate, so that the superposition between the modes of the fundamental wave and the high harmonic wave are lowered, resulting in lowering of the output power of the high harmonic wave. If the output power of the high harmonic wave is lowered, the optical damage is remedied, so that the original condition is recovered. Thereafter, the above-mentioned relationship is repeated, and accordingly, it is considered that the output power of the high harmonic wave becomes unstable. Specifically, 50% of variation in output power occurs when 3.5 mW of the high harmonic wave is produced.
Further, in the case of the conventional method in which a linear layer is applied onto nonlinear optical crystal by sputtering, when the conversion from the TM.sub.00 mode into the TM.sub.01 mode of the high harmonic wave is carried out, the film thickness of the layer formed by the sputtering varies by about 100 nm, which greatly exceeds a tolerance of 10 to 20 nm for the film thickness of the device. Thus, a high degree of efficiency cannot be obtained. As in this example, a linear layer built up by vapor deposition or epitaxial growth is, in general, not uniform, and accordingly, similar results are obtained.